THE HETEROGENEOUS ICELAND PLUME - ND-SR-O ISOTOPES AND TRACE-ELEMENT CONSTRAINTS

We present a comprehensive set of Sr, Nd, and O isotope data and trace element concentrations from tholeiitic and alkaline lavas of the neov olcanic zones of Iceland (picrites, olivine and quartz tholeiites, tra nsitional and alkali basalts, differentiated rocks). Variations in the oxygen isotope results allow us to distinguish two groups. The first, which comprises quartz tholeiites and more differentiated rocks usual ly associated with central volcanoes, has low deltaO-18 values (+5 to +1 parts per thousand) resulting from interaction with the hydrotherma lly altered Icelandic crust. The second group, which contains picrites , olivine tholeiites, and alkali basalts, has normal mantle oxygen iso topic compositions (deltaO-18 = +5 to +6 parts per thousand) which are thought to represent those of the mantle source. Nd isotopic composit ions vary greatly, from Nd-143/Nd-144 = 0.51314 in picrites to 0.51295 in alkali basalts. To produce such a variation for rocks with the che mical compositions of Icelandic volcanics (Sm-147/Nd-144 = 0.12-0.28) requires >200 m.y., a period that greatly exceeds the maximum age of I celandic crust. Previous models, in which the Sr isotopic variations w ere explained in terms of evolution of crustal reservoirs, are invalid ated, and mantle reservoirs with different Nd and Sr isotopic composit ions are indicated. The Iceland data define a linear array in the Sr-N d isotope diagram which overlaps both mid-ocean ridge basalt and ocean ic island basalt fields and indicates mixing between depleted and enri ched end-members. Alkali basalts come preferentially from an isotopica lly and chemically enriched component of the Iceland plume, and picrit es come from a more refractory, more depleted portion. Positive Sr, Rb , and Ba anomalies are present in picrites and other lavas with low tr ace element contents. These anomalies are not correlated with isotopic differences but are nevertheless believed to result from interaction between the parent magmas of these rocks and altered Icelandic crust. This indicates that even the most primitive Icelandic lavas have been contaminated with some crustal material.